Recently, wireless communication technology has developed significantly in the fields of broadcasting and communication and has overcome problems regarding reliability such as momentary disconnection that is specific to wireless. Consequently, application of the wireless communication technology to the fields of control and measurement in which higher reliability is demanded than in the fields of broadcasting and communication is underway.
In the fields of control and measurement, especially, equipment that builds a social infrastructure (hereinafter referred to as “social infrastructure equipment”) is demanded to have, particularly, higher reliability of communication quality and higher reliability of communication equipment, in other words, long-life operations, as compared with general consumer equipment in the fields of broadcasting and communication. Social infrastructure equipment includes, for example, an elevator system which is depicted in FIG. 12 and an transformer equipment control system which is depicted in FIG. 13, among others.
Social infrastructure equipment is overwhelmingly large in size, as compared with general consumer equipment, and is made robustly of metal members. This social infrastructure equipment itself becomes a scatter of electromagnetic waves. Hence, wireless communication in social infrastructure equipment is often performed in an environment where multi-path interference (multi-path waves) generated by scattering interfere with each other. Therefore, it is hoped to achieve highly reliable wireless communication under the environment where interference due to multi-path interference (multi-path waves) occurs.
When a difference in distance between a plurality of electromagnetic waves traveling from a transmitting point until arriving at a receiving point is an odd multiple of a half-wave length, interference of these electromagnetic waves causes their energy toe be cancelled out to zero and makes communication impossible. This problem has heretofore been coped with by a space diversity technique in which multiple antennas are installed such that they are spatially separated by half-wave length from each other. In the space diversity technique, even if the energy of an electromagnetic wave received by one antenna becomes zero by interference, the energy of an electromagnetic wave received by another antenna installed with a distance of a half-wave length from the one antenna, is strengthened by interference, which enables reception by either antenna.
In a wireless communication environment in a social infrastructure, reflections are caused by a distribution of fixtures distributed there which are scatters. When an average distance that the reflected electromagnetic waves travel is comparable with the distance between the antennas (the distance of the half-wave length of electromagnetic waves) for realizing space diversity, it becomes very likely that the energy of electromagnetic waves arriving at the antennas becomes zero due to another interference caused by multi-path reflections. Hence, it becomes difficult to ensure the reliability of wireless communication.
In social infrastructure equipment, there is a possibility that an electromagnetic wave generated by a wireless transmitter is reflected by the social infrastructure equipment itself and turns into multi-path waves (multi-path waves) to go toward a receiver and arrive at the receiver from all directions. Hence, a large number of antennas are needed if the space diversity technique is applied. For example, even if multi-path waves (multi-path waves) are assumed to come only in all directions in a plane, it is necessary to prepare a plurality of arrayed antennas. Since the distance between adjacent antennas is the half-wave length of electromagnetic waves to receive, there is a possibility of exceeding a scale to which antennas can be provided on the social infrastructure equipment.
In Patent Literature 1 (Japanese Unexamined Patent Application Publication No. Hei 10(1998)-135919), particularly, in Abstract and FIG. 3, there is disclosed a technique in which the polarization plane of radio waves is rotated to suppress the influence of fading and noise in wireless communication. Besides, paragraph 0006 of the specification of Patent Literature 1 discloses that “comprises, at a transmitting end, two pairs of dipole antennas arranged to intersect at a right angle and extended perpendicularly to a direction of transmission in order to transmit radio waves, rotating their polarization plane and a transmitting device having dual balanced modulation outputs for exciting these antennas, further comprising, at a receiving end, a receiving device that receives incoming radio waves, detecting their rotated polarization plane”.
In Patent Literature 2 (Japanese Unexamined Patent Application Publication No. Sho 61(1986)-024339), there is described a method that uses two carriers having different frequencies as a first frequency without using the third frequency, makes each carrier carry different pieces of information through the use of a second frequency, transmits these two carriers by using different polarizations, and detects a frequency of a difference between the frequencies of the two carriers as a third frequency at a receiver.